On the ferrite-austenite equilibrium in the Fe-Cu system
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et al. log c a = ---4500/T + 4.335
[1]
c a is the solubility of copper e x p r e s s e d as p e r c e n t by weight. However, this equation i n d i c a t e s that the s o l u b i l i t y in f e r r i t e would i n c r e a s e f a s t e r than the s o l u b i l i t y in a u s t e n i t e a c c o r d i n g to Speich etal. a s the t e m p e r a t u r e is r a i s e d a n d the two s o l u b i l i t i e s would be equal at a t e m p e r a t u r e of a p p r o x i m a t e l y l190~ T h i s r e s u l t is s u r p r i s i n g in view of the following cons i d e r a t i o n . Both s o l u b i l i t y l i n e s r e p r e s e n t e q u i l i b r i u m with the E phase which is a l m o s t pure c o p p e r . The copper a c t i v i t y is thus a l m o s t the s a m e on t h e s e two l i n e s . When c o m p a r i n g the i r o n a c t i v i t y on the two s o l u b i l i t y l i n e s , it mus~ be r e m e m b e r e d that p u r e i r o n has different f r e e e n e r g i e s i n the a and y s t a t e s . However, this d i f f e r e n c e v a n i s h e s at the a/V t r a n s i tion temper~tture (1184.5~ and is v e r y s m a l l in its neighborhood. The effect of copper on the i r o n a c tivity can be well e s t i m a t e d by R a o u l t ' s law for the r e l a t i v e l y low copper c o n t e n t s involved. Of c o u r s e , this effect gives no d i f f e r e n c e between the a and V p h a s e s when they have the s a m e copper c o n t e n t . As a c o n s e q u e n c e , if the s o l u b i l i t y lines for e phase in f e r r i t e and a u s t e n i t e i n t e r s e c t at l190~ it m a y thus be concluded that the point of i n t e r s e c t i o n r e p r e s e n t s f e r r i t e and a u s t e n i t e with a l m o s t the s a m e i r o n a c tivity as well a s copper a c t i v i t y . As a c o n s e q u e n c e , the d i f f e r e n c e in f r e e e n e r g y between f e r r i t e and a u s tenite of the s a m e c o m p o s i t i o n should be v e r y s m a l l at the point of i n t e r s e c t i o n . It s e e m s difficult to r e concile this c o n c l u s i o n with the fact that the shape of the a + Y as well as the 6 + 7 two-phase r e g i o n s i n d i cate that copper s t a b i l i z e s a u s t e n i t e in c o m p a r i s o n with f e r r i t e at a l l t e m p e r a t u r e s . In view of this diff i c u l t y , it s e e m e d to be i n t e r e s t i n g to e x a m i n e the t h e r m o d y n a m i c s of the F e - C u s y s t e m and thus to obt a i n a sound b a s i s for the e x t r a p o l a t i o n of the s o l u b i l i t y of copper in f e r r i t e . Z e n e r 4 has pointed out that the effect of a l l o y i n g e l e m e n t s on the C u r i e t e m p e r a t u r e of f e r r i t e m a y s t r o n g l y influence the t h e r m o d y n a m i c s of the f e r r i t e /
HANS HARVIG,GERNOT KIRCHNER, and MATSHILLERT are Assistant Professors and Professor, respectively, Department of Physical Metallurgy, Royal Institute of Technology, Stockholm, Sweden. Manuscript submitted March 3, 1971. METALLURGICAL TRA
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